1. Field of the Invention
The present invention relates to a photoelectric converting apparatus for converting optical information into electric signals.
2. Description of the Prior Art
Certain photoelectric converting apparatus are provided with groups of converting elements having photoelectric converting performance and a scanning circuit for time-sequentially collecting electric signals from said groups. Such apparatus may be composed, for example, of combination of photodiodes and MOS field effect transistors (hereinafter called MOS type), or of a so-called charge transfer device (CTD) such as a charge-coupled device or a bucket brigade device.
However, in such a known device utilizing the MOS type technology or CTD technology, the light-receiving area in the photoelectric converting part is inevitably limited by the dimension of the single crystal silicon wafer used as the substrate. Since the present technology only obtains a single crystal silicon wafer of several inches in diameter at a maximum in consideration of the uniformity over the entire area, the light-receiving area of such known photoelectric converting apparatus utilizing the MOS type of CTD technology based on such a silicon wafer cannot exceed the above-mentioned dimension.
The photoelectric converting apparatus having a light-receiving area of such limited dimension requires, for example in the application as an input unit for a digital copying machine, the use of an optical system of a high reduction rate between an original to be copied and the light-receiving area for focusing the image of said original onto the light-receiving area.
In such a case the resolving power of the image is encountered by technical limitations as explained in the following.
As an example, in case of copying an original of A4 size with a photoelectric converting apparatus having a light-receiving area with a longitudinal length of 3 cm and with a resolving power of 10 lines/mm, the original image projected onto the light-receiving area is reduced to a size of about 1/69, so that the effective resolving power of the apparatus to said original drops to about 1.5 lines/mm. In this manner the effective resolving power is reduced in inverse proportion to the dimension of the original.
In order to overcome such a disadvantage in the above-mentioned converting apparatus, there is required a production technology for improving the resolving power in the photoelectric converting unit, but such technology also has a limitation since the required resolving power in a limited small area is only achievable with an extremely high degree of integration while avoiding defects in the component elements.
For this reason there is desired a photoelectric converting apparatus having a light-receiving area lengthened in size and improved in resolving power. Also there is proposed a photoelectric converting apparatus having photoelectric converting units on plural chips for projecting, dividedly, the image on said plural converting units, instead of utilizing a large single chip.
As explained in the foregoing because of the use of a large-sized wafer or plural chips, in such an apparatus the photoelectric converting elements formed on different parts of the wafer or on different chips tend to show mutually different characteristics to receive mutually different effects by the external perturbation, thus providing uneven output signals.